It is known that industrial waste water, saline water, and sewage contain large amounts of ions and silica. In addition, in a cooling tower, heat is exchanged between a high-temperature exhaust gas discharged from the boiler, etc., and cooling water. As a result of this heat exchange, some of the cooling water turns into steam, and, accordingly, ions and silica in the cooling water are concentrated. Therefore, the cooling water discharged from the cooling tower (blowdown water) has increased concentrations of ions and silica.
Water containing a large amount of ions is subjected to a demineralization treatment and then discharged into the environment. As devices that perform the demineralization treatment, a reverse osmosis membrane device, a nanofiltration membrane device, an ion-exchange equipment, and the like are known.
Among ions contained in the water mentioned above, monovalent cations such as Na+, K+, and NH4+ and anions such as Cl− and NO3− are highly soluble in water. On the other hand, divalent metal ions such as Ca2+, anions such as SO42− and CO32−, and silica are scale-forming components. Salts and silica of scale-forming components have low solubility in water, and thus they tend to be deposited as scales. In particular, the saline water, industrial waste water, and blowdown water from a cooling tower mentioned above contain large amounts of Ca2+, SO42−, carbonate ions (CO32−, HCO3−), and silica. An example of the property is as follows: pH: 8, Na ions: 20 mg/L, K ions: 5 mg/L, Ca ions: 50 mg/L, Mg ions: 15 mg/L, HCO3 ions: 200 mg/L, Cl ions: 200 mg/L, SO4 ions: 120 mg/L, PO4 ions: 5 mg/L, SiO2 ions: 35 mg/L. Among these, the concentrations of Ca ions, Mg ions, SO4 ions, and HCO3 ions are high, and as a result of their reaction, scales (CaSO4, CaCO3, etc.) are formed. In addition, depending on the concentration percentage, silica components present in waste water also serve as scale components adhering to the instrument, etc. When scales are produced in the device that performs a demineralization treatment, the treatment capacity is reduced. Therefore, it is required to perform a demineralization treatment without allowing for the production of scales.
Here, examples of plants using a water-cooling-type cooling tower are plants equipped with power generation facilities (power generation facilities include those for business purposes for electric power selling and those for industrial purposes for in-house electricity use, and the power generation is thermal power generation, geothermal power generation, etc.), plants equipped with power generation facilities and cooling facilities, etc. In addition, plants include ordinary chemical plants, steel plants, mining plants, oil field plants, gas field plants, mechanical plants, etc.
As a process for removing Ca ions, a lime soda process is known. According to the lime soda process, sodium carbonate is added to water to be treated, and Ca ions in the water to be treated are deposited/precipitated as calcium carbonate and thereby removed from the water.
Patent Literature 1 discloses a waste water treatment device including a combination of a chemical softening device, an ion-exchange equipment, a reverse osmosis membrane device, and the like using the lime soda process.